The thermoplastic resin industry processes approximately 7.75 million tons of thermoplastic resin per year. The machinery used to process these resins, injection molding machines and extruders, are subject to substantial wear and tear.
Specifically, injection molding machines and extruders operate at high temperatures, and usually, at some point during the resin processing cycle, under high pressures. In the injection molding process, the plastics are either melted into a viscous liquid, or softened to a putty like solid, and are forced, with pressure, into the mold. The mold is then usually allowed to cool, and the resin re-solidifies, only now the resin pellets are a solid mass the shape of the mold.
Extruders consist of a tube, or barrel, that contains an auger, or screw device. The barrel is heated, so that as the plastic pellets travel down the tube, through the action of the screw, they soften into a very flowable putty which exits the barrel through an opening that is the shape of the final product being produced. As the softened plastic exits the barrel, into the open air, the extruded shape instantly hardens.
When organic molecules are subjected to the temperatures (300.degree.-500.degree. F.) and pressures (up to several tons per square inch) associated with the extrusion and molding processes, there is some tendency for degradation of the molecules to occur. Although antioxidants and heat stabilizers are usually added to the plastics, small amounts of the plastic degrade and the residue particles plate out onto the surfaces of the feed lines and mold surfaces of injection molding machines, and onto the screw and barrel surfaces of extruders. Over time, the residue gradually builds up into a baked on blackish-brown film. The film eventually grows to a thickness where it begins to either interfere with the process causing deformation of parts or extrusions, or flakes off into the hot liquid or putty like plastics, and becomes a surface defect. When the carbon black film builds up to this critical thickness, where it becomes a major quality problem, the processing equipment has to be cleaned.
Other occasions that facilitate the cleaning of plastics processing equipment are when either a resin color change is made or when a resin with compounded ingredients and fillers, follows in a production run, a resin that contains different fillers or additives that should not be intermixed with the new resin system. A thorough cleaning of the injection molding machine or extruder is required, for both of these cases, before the next production run can begin.
Currently, a number of methods are employed to clean plastics processing equipment. One method is to disassemble the injection molding machine or extruder, and have the metal parts submerged into a hot caustic bath. Usually these baths contain some surfactants also. The caustic, over time, breaks up the carbon black build up. This cleaning process is the least desirable, of the potential cleaning methods, as the time involved with disassembling, dip cleaning, and re-assembling the equipment adds up to substantial loss of production time. Also, there are serious safety considerations whenever hot caustic baths are employed.
Another cleaning method employed, usually when color changes or compound changes occur, is to simply run the mold or extruder through multiple cycles using the new color or compound. The cycle is repeated with the new compound until parts are made or extrusion occurs which are free from the previous color or filler additives. The drawback of this cleaning alternative, is that the generation of large amounts of scrap can occur. Often times this scrap cannot be chopped up or re-ground for re-use, and ends up being landfilled.
Cleaning of extruders or injection molding machines, is also accomplished with specialty formulated resin based "cleaning compounds." These cleaning compounds are solid thermoplastic resins that contain surface active agents, abrasive fillers (usually glass or finely chopped fiberglass), and sometimes an amine compound such as monoethanolamine. The cleaning resins are put into the plastics processing equipment in the same manner as compound resins for making production parts. The equipment is operated as if normal production is occurring, except that: (a) the equipment is operated at a slower rate, and (b) the equipment is occasionally shut down. The slower operating rate allows for the abrasive fillers to "grind" away at any hardened build-up of carbon, and the occasional stopping of the equipment gives the monoethanolamine a chance to work at dissolving carbon build-up. Carbon deposits are normally susceptible to alkaline or basic agents, such as monoethanolamine. Cleaning compositions are disclosed in U.S. Pat. Nos. 5,139,694; 5,443,768; 5,427,623; 5,397,498; 5,395,456; 5,298,078; 5,238,608; 5,236,514; 5,124,383; 5,108,645; 5,087,653; and 4,838,945. Specifically, Itoh (U.S. Pat. No. 5,298,078) teaches the melting of polystyrene and polyethylene and addition of alkaline salts and glass fibers as cleaning ingredients. Kmiec (U.S. Pat. No. 5,139,694) and Obama (U.S. Pat. No. 5,124,383) teach the melting of polyethylene resin (substituted pyrrolidones are not soluble in polyethylene or polypropylene) and then adding in abrasive inorganic fillers and polyethylene waxes and fatty acid amide waxes. Abrams (U.S. Pat. No. 5,395,456) teaches the melting of polymers and inclusion of calcium carbonate abrasive, and rosins as cleaning ingredients.
Scheilbelhoffer (U.S. Pat. No. 5,443,768) and Obama (U.S. Pat. No. 5,108,645) both disclose the melting of polymers and the inclusion of hard methacrylate and acrylate compounds as cleaning media. Also, Ishida (U.S. Pat. No. 5,397,498) discloses the melting of a thermoplastic and inclusion of polyalkylene oxide based polyol cleaning agents.
An alternative method for making an extruder cleaning compound, is taught by Schumann (U.S. Pat. No. 5,427,623)(U.S. '623). U.S. '623 teaches a method for making an Acrylonitrile Butadiene Styrene (ABS) powder (or resin) cleaning compound which contains water. The ABS cleaning resin is produced by taking an ABS emulsion and separating the ABS polymer from the emulsion by adding an ionic salt to the emulsion, in large enough quantities to cause the emulsion to break.
A major drawback to these cleaning compounds is that the resins themselves tend to be abrasive in nature. Acrylate based resins that require high temperatures for melting, to flow state, are normally used, and can be very abrasive on the metal surfaces of the equipment. This extra wear on the surface can detract from the useful life of the equipment. Another drawback of some of these cleaning resins is that they contain monoethanolamine which has a high degree of toxicity associated with it. During that portion of the cleaning cycle when the processing equipment is shut down, large quantities of the amine compound vapors are emitted from the hot machines.
An improvement in the art of cleaning plastics processing equipment is needed. The process should not require the disassembling of equipment to accomplish proper cleaning, and should not require the use of highly alkaline compounds or abrasive resins.
The present invention is just such an improvement in the art. Applicants have surprisingly discovered a novel plastics processing equipment cleaning compound comprising a bead or pellet having a hard thermoplastic resin outer core and a soft inner core containing alkyl or alkoxy substituted pyrrolidones. Applicants' novel cleaning compound is added directly into extruders and injection molding machines in the same way that resins are fed during routine production, eliminating the need for disassembling the machines. As the cleaning compound is melted and forced through an extruder or injection molding machine's interior, it softens and removes carbon build up and hardened resins from the surfaces of the machine's barrel and screw.